The present invention relates to a reception method and reception circuit in a wireless communication system employing CDMA (Code Division Multiple Access), and in particular, to a reception method and reception circuit that employ a RAKE combining receiver.
CDMA is one multiple access technique in wireless communication systems. The application of CDMA in a mobile communication system made up by a base station and a plurality of mobile communication terminals has the advantages of both allowing an increase in the number of terminals that can be accommodated by the system and a reduction of the transmission power.
Spread-spectrum modulation is generally used in communication systems that employ CDMA. In communication by spread-spectrum modulation, the transmission signals are subjected to spreading modulation using a PN (Pseudo-random Number) code as a spreading code on the transmitting side to spread the spectrum of transmission signals, and these signals are then sent to the receiving side. On the receiving side, the demodulated transmission signals are obtained by establishing synchronization and then subjecting the received signals that have a spread spectrum to despreading using the same PN code as was used on the transmission side. This PN code that is the same as was used on the transmission side is referred to as the xe2x80x9cspreading code replica.xe2x80x9d
Although direct sequence is used as the method of spread-spectrum modulation in the following explanation, the same discussion holds true if a frequency-hopping spread modulation or the like is employed.
Radio waves that arrive at a receiving station from a transmitting station include, in addition to direct waves, i.e., the component that is propagated directly from the transmitting station to the receiving station, components that are reflected by, for example, mountains, the ground surface, and buildings, and reach the receiver by different propagation paths. The number of radiowave components that pass over different paths are equal in number to the different propagation paths, and the time for each radiowave component to arrive at the receiving station differs according to the length of path taken to arrive at the receiving station. These radiowave components that pass over different paths are referred to as the multipath components. Collecting and combining these different radiowaves having different arrival times while giving each a delay time according to the time to arrive would allow addition of these signals to the received signal of the direct wave, thereby enabling a larger received signal than for the direct wave alone, an improvement in S/N (signal-to-noise ratio), and with this improvement in S/N ratio, a reduction in transmission power.
The direction of arrival of a radiowave component as seen from the receiver differs for each propagation path, and the technique of increasing reception sensitivity by receiving and combining radiowaves that arrive from many directions is called RAKE combining because a visual representation would be similar to a rake.
FIG. 1 is a block diagram showing the configuration of a conventional receiver used in a CDMA communication system that employs RAKE combining, i.e., a RAKE receiver. Although here referred to as a receiver, this is actually a mobile terminal that communicates with a base station in a CDMA mobile communication system.
This receiver is provided with: antenna 222, transmission/reception filter 221 connected to antenna 222 for separating transmission signals and received signals; reception radio-frequency unit 201 connected to the receiving-side port of transmission/reception filter 221 for amplifying and frequency-converting radio-frequency received signals received at antenna 222 and converting it to received signals on the baseband; a plurality of baseband reception units 202 that receive baseband received signals in parallel from reception radio-frequency unit 201 and perform despreading of received signals using a prescribed PN code; power combiner 204 that combines signals after despreading that are outputted from each of baseband reception units 202; reception speech processor 206 for decoding the combined signal to a speech signal; ear receiver 207 for outputting speech after processing; and arrived radiowave search circuit 203 that performs a search of arrived radiowaves of each propagation component for carrying out RAKE combination and that reports the timing of despreading for each of baseband reception units 202. The example shown in the figure is provided with six baseband reception units 202. In this case, reception radio-frequency unit 201, baseband reception units 202, arrived radiowave search circuit 203, power combiner 204, reception speech processor 206, and ear receiver 207 constitute a reception unit. This receiver is further provided with a transmission unit that is made up of: microphone 210 that converts input speech to an electronic signal (speech signal), transmission speech processor 211 that encodes the speech signal that is outputted from microphone 210, baseband transmission processor 213 that spread-modulates the encoded signal by a prescribed PN code and converts it into baseband transmission signals; and transmission radio-frequency unit 214 that converts baseband transmission signals to radio-frequency transmission signals. The output of transmission radio-frequency unit 214, i.e., radio-frequency transmission signals, are applied to the transmission-side port of transmission/reception filter 221.
Since the arrival times of radiowaves differ for differing propagation paths, the receiver shown in FIG. 1 not only has a plurality of baseband reception units 202 for despreading the baseband signals, but uses arrived radiowave search circuit 203 to find the arrival time of each component having a different propagation path. Arrived radiowave search circuit 203 then reports the arrival time (delay time) of the radiowave component of each propagation path to a respective baseband reception unit 202, and each baseband reception unit 202 carries out despreading of the received radiowaves while shifting the timing of the PN code in accordance with the reported arrival time. Since the timing of the PN code is shifted according to the arrival times, the signals that are outputted from each of baseband reception units 202 after despreading have matched phase, and combination of the power of these signals at power combiner 204 enables a larger received signal. In a RAKE receiver, the component that carries out despreading for each arrival time is called a xe2x80x9cfinger.xe2x80x9d The above-described RAKE receiver has six baseband reception units 202, and thus has six fingers.
The chip rate of the PN code that is used for spread modulation on the transmission side and despreading on the receiving side may be, for example, 4 MHz, in which case the time per chip is 0.25 xcexcs. In contrast, the difference in arrival times of the multipath components may reach several tens of microseconds. FIG. 2 is a graph showing the relation between the delay times (the amount of shift in chip phase) and the received power after despreading if the chip phase of the PN code used in despreading is shifted by degrees, for example, by xc2xc chip, in an environment in which the radiowaves on different propagation paths are received at the same time. This graph is called a delay profile.
Since radiowaves that arrive from a transmission station by different propagation paths will have different arrival times, peaks will occur in the received power at different delay times. The example shown in the figure has three peaks, peak #1, peak #2, and peak #3, and the spacing of these three peaks corresponds to the difference in arrival times. Arrived radiowave search circuit 203 searches for the positions of these peaks and assigns one peak to each of baseband reception units 202. The search for the positions of peaks in this case is equivalent to finding differences in arrival times. In each of baseband reception units 202, the radiowave component that has arrived by the propagation path that corresponds to the assigned peak undergoes despreading by carrying out despreading of the received signal by the PN code of the chip phase that corresponds to the assigned peak position. In this way, the received signals of the radiowave components of each of the different propagation paths undergo appropriate despreading at each baseband reception unit, and the despread received signals are power-combined to enable an increase in the intensity of the received signal and an improvement in the S/N.
FIG. 3 is a block diagram showing an example of the configuration of the arrived radiowave search circuit in the conventional RAKE receiver. This arrived radiowave search circuit is provided with: despreading unit 231 that is given a PN code and that performs despreading of received signals based on the PN code; integrator 232 that integrates the output signal from despreading unit 231 over the time interval of one symbol and integrates over a still longer period; dual-port RAM (DPRAM) that stores the output values of integrator 232; searcher circuit 235 that searches inside dual-port RAM, detects peaks for each delay time, and assigns peak positions for each of baseband reception units 202; PN code generator 236 that generates the PN code; and control unit 237 that both controls the phase of the PN code that is generated by PN code generator 236 and controls the write addresses of dual-port RAM 234. The despreading of received signals is then performed in despreading unit 231 while shifting the phase of the PN code that is generated at PN code generator 236 by increments of, for example, xc2xc chip; the output from despreading unit 231 is integrated at integrator 232; and the value of the integration result is stored in dual-port RAM 234 in correspondence with the phase of the PN code at that time. A delay profile such as is shown in FIG. 2 is thus stored in dual-port RAM 234. Searcher circuit 235 searches inside this dual-port RAM 234, determines a peak position that is to be assigned to each of baseband reception units 202, and reports to each baseband reception units 202 an arrival time difference that corresponds to a determined peak position.
In a mobile communication system such as an automobile telephone system or portable telephone system, at least one of a transmission station and receiving station can be assumed to be moving, thereby causing the propagation paths of radiowaves between the transmission station and receiving station to change minute to minute and the positions of peaks that correspond to propagation paths to also change as a result. In some cases, a propagation path may disappear and the associated peak may vanish, or a new propagation path may occur and a new peak also appear as a result. In particular, a delay profile may change abruptly when a mobile unit is moving at high speed, and the peaks of arrived radiowaves must constantly be searched to prevent loss of the peak positions.
As an example, Japanese Patent Laid-open No. 181704/97 (JP, 09181704,A) discloses a multipath search method and CDMA signal receiver that can follow changes in the delay profile of a CDMA signal and perform RAKE combination of a plurality of propagation paths. In this method, a receiver is used that includes a search finger and tracking fingers that are similar to the above-described baseband reception unit. As shown in FIG. 4, the levels of received signal are detected at all chip phases by the search finger in Step S1 as the initial operation. Based on the mean received signal level that is detected in this initial search, paths that are to undergo RAKE combining are selected in Step S2, and the phases of these selected paths are detected using the tracking fingers. After performing integration and damping, demodulation is performed for each path, following which RAKE combining is carried out. Each tracking finger has the function of independently tracking each path. When paths overlap, i.e., when the same peak is assigned to tracking fingers, a selected path is reassigned to one tracking finger based on ranking information of the received signal levels in Step S4. On the other hand, the search finger shown in Step S3 detects the received signal levels for all chip phases of the range of delay times that are to undergo RAKE combining. An average is taken for each chip phase and RAKE combining paths are selected at a fixed period, and a corresponding spreading code replica code (PN code) is then given to each tracking finger.
FIG. 5 is a block diagram showing the configuration of the CDMA signal receiver disclosed in the above-described Japanese Patent Laid-open No. 181704/97. This receiver is provided with a plurality of tracking fingers 300, search finger 350, long code spreading code replica generator 381 that generates a PN code, RAKE combining path selector 382 for selecting paths that are to undergo RAKE combining; detectors 383 that each perform absolute synchronous detection of the output of a respective tracking finger 300; and RAKE combining circuit 385 that performs RAKE combining of the output of each detector 383 and outputs as an output signal. A received input spread signal is supplied to each tracking finger 300 and search finger 350.
Since each tracking finger 300 is provided with an independent tracking function as described in the foregoing explanation, two sets of multipliers 301 and 302 and integration-damping circuits 304 and 305 are provided for tracking in addition to multiplier 303 and integration-damping circuit 307 that are provided for signal reception. Multipliers 301 to 303 are for multiplying a PN code with a received input spreading signal and performing despreading, and integration-damping circuits 304, 305, and 307 are for integrating the outputs of multipliers 301 to 303 at fixed time intervals. Amplitude squaring circuits 308 and 309 are provided for squaring each of the outputs (amplitudes) of integration-damping circuits 304 and 305 for tracking. In addition, each tracking finger 300 is provided with: subtractor 310 for calculating the difference in outputs of amplitude squaring circuit 308 and 309; loop filter 311 for receiving the outputs of subtractor 310; spread code replica timing control signal generator 312 for determining the amount of delay (timing) of the PN code based on the output of loop filter 311; and spreading code replica delay unit 306 for giving a delay, for each of multipliers 301 to 303, to a PN code from long code spreading code replica generator 381 based on control signals from spreading code replica timing control signal generator 312 and outputting to these multipliers 301 to 303.
Search finger 350 is provided with: multiplier 351 for multiplying the received input spreading signal with the PN code; integration-damping circuit 352 for integrating the output of multiplier 351 at fixed intervals; amplitude squaring circuit 353 for calculating the square of the output (amplitude) of integration-damping circuit 352; reception level memory 354 for storing the delay profile based on the output of amplitude squaring circuit 353; and spreading code replica delay unit 355 for giving a delay to the PN code from long code spreading code replica generator 381 and supplying the delayed PN code to multiplier 351.
As described hereinabove, the delay profile of the received signals must be obtained and the peak positions detected when performing RAKE reception in a mobile communication system. In the case of the circuit shown in FIG. 1, the arrived radiowave search circuit must continue to search for peaks over the entire conceivable range of variation of arrival time differences for each multipath component, and current is therefore constantly consumed. Similarly, in the case of the signal receiver described in Japanese Patent Laid-open No. 181704/97, not only must the search finger be operated constantly, but the circuits for tracking at each tracking finger must also be constantly operated, and the consumed current therefore increases accordingly. In a receiver that performs RAKE combining, the circuits that follow output of the reception radio-frequency unit are typically configured to perform digital signal processing, and the computation load of the integrating process for the peak search is greater than for the integration process for despreading of normal signals and therefore consumes proportionally more power. Moreover, the process of searching dual-port RAM or received level memory to find peak positions entails a process of step-by-step comparison of the data inside this memory and this also leads to greater power consumption.
A search of arrived radiowaves that is performed by the conventional method to realize RAKE reception as described hereinabove results in a high level of power consumption, and in particular, raises the problem of reducing the possible talk time of a mobile terminal that presupposes the use of a battery drive, i.e., in an automobile telephone terminal or portable telephone terminal.
It is therefore a first object of the present invention to provide a reception method that can shorten the time of operation of an arrived radiowave search circuit in a mobile terminal that performs RAKE combining and thus reduce consumption of current.
It is a second object of the present invention to provide a reception circuit that can shorten the time of operation of an arrived radiowave search circuit in a mobile terminal that performs RAKE combining and thus reduce consumption of current.
The first object of the present invention is achieved by a reception method in a mobile terminal that realizes communication by Code Division Multiple Access (CDMA) using a spread spectrum, that performs a search of arrived radiowaves to detect radiowaves of propagation paths having different delay times, and that uses RAKE combining to combine received signals from radiowaves of different propagation paths; wherein the reception method is a CDMA reception method comprising the steps of detecting the reception state of received signals, and changing the range of delay time in which a search of arrived radiowaves is performed when the reception state satisfies a prescribed condition.
The first object of the present invention is also achieved by a reception method in a mobile terminal that realizes communication by Code Division Multiple Access (CDMA) using a spread spectrum, that performs a search of arrived radiowaves to detect the radiowaves of propagation paths having different delay times, and that uses RAKE combining to combine received signals from radiowaves of different propagation paths; wherein the reception method is a CDMA reception method comprising the steps of detecting the amount of fluctuation in the received electric field of the received signals, and changing the range of delay time in which a search of arrived radiowaves is performed when the amount of fluctuation satisfies a prescribed condition.
In the CDMA reception method according to the present invention, the amount of fluctuation in the received electric field is typically the fading pitch or fading depth. In addition, a method can be proposed as the method of limiting the range of delay time in which, for example, the reciprocal of the fading pitch is compared with a prescribed threshold value, or the speed of movement of the mobile terminal is calculated from the fading pitch and this speed of movement is compared with a threshold value, and if the threshold value is smaller, a peak search is carried out only in, for example, the vicinities of previously found peaks, i.e., the vicinities of peaks found in a previous search.
The second object of the present invention is realized by a CDMA reception circuit, which is a reception circuit in a mobile terminal that realizes communication by Code Division Multiple Access (CDMA) using a spread spectrum and that uses RAKE combining to combine received signals from radiowaves of different propagation paths; that comprises: baseband reception means that performs despreading of received signals, electric field intensity measurement means that detects the amount of fluctuation in the received electric field, and an arrived radiowave search circuit that performs a search of arrived radiowaves that detects the radiowaves of propagation paths of different delay times and assigns radiowaves of different propagation paths to respective baseband reception means; and in which the range of delay times of the search of arrived radiowaves that is performed by the arrived radiowave search circuit is modified when the amount of fluctuation satisfies a prescribed condition.
In the CDMA reception circuit of this present invention, the baseband reception means is typically made up from a plurality of baseband reception units required for RAKE reception, i.e., a plurality of fingers.
The delay time of a radiowave on each propagation path changes with movement of the mobile terminal, but it can be assumed that change in the arrival times will be higher at high speeds of movement, and lower at low movement speeds. When the change in the arrival time is low, peaks should be searched only in the vicinities of previously found arrival times, and there is no need for the arrived radiowave search circuit to continue to search for peaks over the entire conceivable range of change in chip phase. Alternatively, the search for arrived radiowaves can be performed intermittently.
In the case of a mobile terminal, fading is observed with movement. The time interval between troughs in the received electric field due to fading is referred to as fading pitch, and fading pitch is dependent on the speed of movement of the mobile terminal. In the present invention, the fading pitch is measured, and this measured fading pitch is compared with a prescribed threshold value, or the speed of movement that is estimated from the measured fading pitch is compared with a prescribed threshold value. The speed of movement of the mobile terminal is determined to be low if the threshold value falls below the fading pitch (in other words, if the threshold value is greater than the reciprocal of the fading pitch), or if the threshold value is greater than the speed of movement. If the speed is determined to be low, the search of arrived radiowaves is performed only in the vicinities of previously found arrival times for each propagation path. This configuration allows the arrived radiowave search circuit to be operated intermittently and can realize a reduction in the power consumption of the entire mobile terminal. Instead of determining if the speed of movement of the mobile terminal is high or low from the fading pitch, the present invention also allows determining whether fading is observable or not. If there is no fading, it can then be determined that the mobile terminal is actually not moving, i.e., is stationary, and in this case the search range of the arrived radiowave search circuit can be drastically limited to realize a still greater reduction in power consumption. In this case, the absence of fading means that, for example, fading is substantially not observable.